The objective of this research is to understand the mechanism(s) by which S-phase specific genes are regulated in normal and SV40- infected cells. Since the expression of these genes is correlated with the growth state of the cell, it is hoped that an understand of their regulation will contribute to an understanding of the control of cell growth in normal and virus-infected cells. This understanding will in turn be important to the study of oncogenic transformation, since one of the properties of transformed cells is that they fail to regulate their growth in a normal way. The approach taken to investigating the regulation of S-phase specific genes will be a molecular one, using the thymidine kinase (TK) gene as a model system. TK expression is regulated during the cell cycle, being low in G0 and G1 phase cells, and increasing as the cells enter S phase. Both genomic and cDNA TK clones will be used to construct a series of mutant and hybrid TK genes. These altered genes will then be transfected into tissue culture cells, and the regulation of their expression will be examined. Using these techniques, the following questions concerning TK regulation will be addressed: 1) At what levels does regulation of the gene occur (transcriptional/post-transcriptional, etc.)? 2) What TK DNA sequences are responsible for this regulation? 3) What is the role of viral proteins in inducing the TK gene? 4) What cellular factor or factors are involved in inducing the TK gene?